Natural-abundance carbon-13 Fourier Transform nuclear magnetic resonance spectroscopy yields resolved resonances of individual nonprotonated aromatic carbons of small native proteins in solution. The observed single-carbon resonances are used to study: (1) Specific chemical modifications of aromatic residues. (2) Titration behavior of tyrosines and histidines. (3) Motions of aromatic side-chains. (4) Conformational changes. (5) Self-association. (6) Environment of the copper in "blue" copper proteins. BIBLIOGRAPHIC REFERENCE: R. S. Norton and A. Allerhand, Studies of Chemical Modifications of Proteins by Carbon-13 Nuclear Magnetic Resonance Spectroscopy. Reaction of Hen Egg-White Lysozyme with Iodine, J. Biol. Chem., 251, 6522-6528 (1976).